I'm going to caveat this by stating up front that obviously HN's source code is not public so I don't know what your hot path looks like, and that I'm not a domain expert on garbage collection, but I do write a fair amount of lisp for SBCL.

Immix-style collectors, like the new GC in SBCL, only compact on an opportunistic basis and so you get fragmentation pressure under load. In that situation, you might be well under the dynamic space size cap but if it can't find a large enough contiguous chunk of free heap it will still die.

So, fragmentation would be my prime suspect given what you described.